On the Inside Pyrroloquinoline Quinone: A New Plant Growth Promotion Factor

Plant growth-promoting rhizobacteria (PGPR) are bacteria that colonize plant roots and enhance plant growth by a wide variety of mechanisms. The use of PGPR in sustainable agriculture is steadily increasing and offers an attractive way to replace chemical fertilizers, pesticides, and supplements. PGPR have been applied to various crops to enhance growth, seed emergence, crop yield, and disease control, and some have been commercialized. A PGPR called Pseudomonas fluorescens B16 was previously isolated from the roots of graminaceous plants. The wild-type B16 colonizes the roots of various plants and produces an antibacterial compound that is effective against certain plant root pathogens. This PGPR strain also significantly promotes the growth of cucumber (Cucumis sativus) and barley (Hordeum vulgare) under greenhouse and field conditions. In this issue, Choi et al. (pp. 657–668) report that a key factor involved in the promotion of plant growth by P. fluorescens B16 is pyrroloquinoline quinone (PQQ). Analysis of culture filtrates confirmed that the wildtype B16 produces PQQ, whereas the mutants defective in plant growth promotion do not. Application of the wildtype B16 on tomato (Solanum esculentum) plants cultivated in a hydroponic culture system significantly increased the height, flower number, fruit number, and total fruit weight, whereas none of the strains that were unable to produce PQQ promoted tomato growth. Furthermore, 5 to 1,000 nM of synthetic PQQ conferred a significant increase in the fresh weight of cucumber seedlings, confirming that PQQ is a plant growth promotion factor. Treatment of cucumber leaf discs with PQQ and the wild-type B16 resulted in the scavenging of reactive oxygen species and H2O2, suggesting that PQQ acts as an antioxidant in plants.